1. Field of the Invention
The present invention relates to a high frequency tuner module and a tuner module, and more particularly a high frequency tuner module which can be integrated on a portable device and a tuner module including the high frequency tuner module.
2. Description of Related Art
In a tuner for receiving television signals, AM/FM broadcast signals and the like, a tuner module is configured with a single layer circuit board where a wiring pattern of signal lines, GND lines, etc. and circuit components are provided on both faces or one face of the board. Lately, in order to reduce size and thickness, enhance mounting density, etc. of the tuner module, tuner modules which use a multilayer board, formed by laminating a plurality of layers, have been developed in which circuit components, signal lines, etc. are placed on a surface of the top layer or inside the multilayer (for example, Japanese Patent Application Laid-open Publication No. 5-327230, hereinafter referred to as Patent Document 1).
A multilayer board preferably used in a tuner module is a board in which each of the laminated layer is formed by a board of low temperature co-fired ceramics (LTCC) whose base material is alumina.
As shown in FIG. 6A, a multilayer board comprising a tuner module M is formed by laminating a top layer L1 (also referred to as first layer L1), a second layer L1, a third layer L3, a fourth layer L4, and so on in this order. In the present specification, a surface of the tuner module M, i.e. a face of the top layer L1 open to outside is referred to as a top layer surface S1, and a signal line or the like formed in an area between the top layer L1 and the second layer L2, such as a signal line SL in the figure, is referred to as a signal line or the like formed on the second layer L2.
According to this representation, for example a GND line GL as shown in FIG. 6A is described as the GND line GL formed on the third layer L3. Also, in this case, the GND line GL is described as being formed on a layer directly below a layer on which a signal line SL is formed.
When high frequency tuner modules for receiving signals in high frequency bands such as television tuners for terrestrial digital broadcasting, tuners for wireless LAN or tuners for satellite radios in practice in the U.S. are configured with multilayer boards as described above in order to reduce the size of the module to be integrated on portable devices, there is a tendency that a loss of signal gain which occurs between the signal lines, the GND lines, and the circuit components become significant.
In a high frequency tuner module M, loss is compared between a case as shown in FIG. 6A, in which a signal line is provided on a layer L2 of the multilayer board laminated with layers L1, L2, L3, L4 and so on with a thickness of, for example, about 50 μm, and a solid GND line GL is provided on the layer L3 directly below the layer L2, and a case as shown in FIG. 6B, in which a solid GND line GL is provided on the layer L4 which is one layer apart from the layer L2 formed with the signal line SL. As shown in the graph in FIG. 7, as the frequency becomes higher, the loss becomes greater. Further, the loss becomes greater when the distance between the lines is small as in FIG. 6A than when the distance between the lines is large as in FIG. 6B.
It is considered this is because a section including a layer of an insulator and two lines bears a characteristic of a condenser and this section functions like a low pass filter with a condenser connecting the two lines. Thus, loss of signal gain occurs by the same mechanism as the insertion loss of a low pass filter.
The capacitance of the condenser is represented by the following formula, where a capacitance of the condenser is C, a permittivity of the insulator is ε, an area of the facing conductors is S and a distance between the facing conductors is D,C=ε×S/D  (1)
Therefore, as described above, the smaller the distance between the lines become, the larger the capacitance C becomes. As a result, it is considered, the loss shown in curve A in FIG. 7, in which the distance between the lines is close, is greater than that of curve B, in which the distance is far.
The loss of signal gain does not only occur between lines inside the multilayer board as shown in FIG. 6A and FIG. 6B, but also may occur between wires placed on the top layer surface S1 of the multilayer board, and between the lines and the circuit components. Therefore, in a multilayer board which carries lines such as signal lines and GND lines and circuit components on the top layer surface S1 as shown in patent document 1, in order to reduce the loss of the signal gain, there needs to be at least a certain distance between the lines and between the lines and the circuit components on the top layer surface S1.
As described above, the multilayer board described in patent document 1 has the circuit components and the lines both placed on the top layer surface, thus the area of the top layer surface needs to be large enough to leave a certain distance between the circuit components and the lines. Therefore, with the multilayer board of the patent document 1, the area of the top layer surface cannot be smaller than a predetermined area, and the size of the tuner module, etc. as described above may not be sufficiently reduced.